Assembly and Method for Measuring a Substance Concentration in a Gaseous Medium by Means of Absorption Spectroscopy

ABSTRACT

An assembly and a method for measuring a gas concentration by means of absorption spectroscopy, in particular for capnometric measurement of the proportion of CO 2  in breathing air in which IR light from a thermal light source is guided through a measuring cell with a gas mixture to be analyzed, and the concentration of the gas to be measured that is contained in the gas mixture is determined by measuring an attenuation of the light introduced into the measuring cell caused by absorption by the gas to be measured. The thermal light source is designed as an encapsulated micro-incandescent lamp with a light-generating coil.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of and claims priority toPCT Application No. PCT/EP2018/053663, filed Feb. 14, 2018, which is aPCT Application of and claims priority to EP patent application number17160250.1, filed on Mar. 10, 2017, the entire contents of which areincorporated herein by reference.

FIELD

The invention relates to an assembly and a method for measuring a gasconcentration by means of absorption spectroscopy, in particular forcapnometric measurement of the proportion of CO₂ in breathing air.

BACKGROUND

The basic principle of such measurements is known. Light, or infraredlight (IR) from a thermal light source in the case of measuring CO₂, isguided through a measuring cell with a gas mixture to be analyzed, andthe concentration of the gas to be measured that is contained in the gasmixture is determined by measuring an attenuation of the lightintroduced into the measuring cell from being absorbed by the gas to bemeasured.

The attenuation of the emitted light depends exponentially on theconcentration, or respectively density of the absorbent gas, thewavelength-dependent absorption coefficient of the gas, and the lengthof the measuring path in which the light crosses the gas to be measured.The measurement of the attenuation of the light by absorption requiresthe knowledge of the amount of the light radiated into the measuringcell and coupled out of the measuring cell. For this reason, twomeasurements are always needed, i.e., on the one hand the measurement ofthe amount of light of the light attenuated by being absorbed in themeasured gas after passing through the measuring cell and, on the otherhand, the measurement of a quantity as a reference measurement which isrepresentative of the radiated amount of light.

This reference measurement can be a direct measurement of the lightintensity or light quantity at the light source, wherein a portion ofthe light from the source is detected which is not used for measuringgas but is nonetheless proportional to the amount of light for gasmeasurement. The advantage of this is that a direct measurement of theradiated light intensity is problematic since the light used forreference measurement is only proportional and not identical to thelight used for the gas measurement. The proportional dependency canchange over the course of time since the light for the referencemeasurement has not passed through the same optical path as the lightfor absorption measurement and accordingly does not possess additionalabsorptions, for example from impurities. Developments over the longterm such as increasing soiling of the measuring window of the measuringcell can also cause a systematic drift of the absorption measuringresults.

An alternative assembly uses reference light that also passes throughthe measuring cell but lies within a wavelength range in which the gasto be measured is scarcely or not absorbed. The reference lighttherefore only undergoes the absorption that is caused by impurities orother systematic sources, and to which the light within the wavelengthrange of absorption of the gas to be measured is also subjected. Withthe exception of wavelength-dependent, i.e., dispersive, effectsincreasing soiling in the measuring cell for example then affects theabsorption measurement in the same way as the reference measurement. Inthe comparison of the two measurements, these contributions largelycancel each other out so that corresponding systematic effects thatnegatively influence the measuring results are largely minimized.

In the field of capnometry, infrared light is used since the gas CO₂ hasa strong absorption band at approximately 4.26 μm, i.e., within themiddle infrared range. The present invention is therefore particularlyapplicable in the field of capnometry but can also be used for otherapplications in which absorption measurements, in particular within theinfrared range, are performed. Capnometry is a monitoring procedure usedin medicine and provides information on the patient's status in thefield of emergency services, in clinics, at-home care and sportsmedicine.

Capnometers are generally small mobile devices, sometimes with a unitthat can be handheld for parameterizing and visualizing, wherein in manycases, the sensor unit is also contained in the manual device in sidestream applications. Such capnometers can also be integrated asready-to-use assemblies in larger devices such as ventilators, metabolicmonitors, inter alia.

Capnometers detect the CO₂ curve of the respiration of a patient. Thismeasurement is performed either in the so-called main stream method orin the so-called side stream method. In the main stream method,generally a sensor is placed in the proximity of the breathing mask ofthe patient that measures the CO₂ concentration through a cuvette in thebreathing hose. In the side stream method, a cuvette with a thin suctionhose is attached in the breathing gas channel, part of the breathing airis conveyed through the suction hose to a sensor module and measuredthere with a certain dead time.

Given the applications, especially in the field of emergency servicesand home care as well as sports medicine, sensor modules should besmall, light and robust in order to withstand vibration, and have a lowpower consumption because mobile use without an external power supply isfrequently necessary. Currently available main stream sensors only havea battery life of 4 to 6 hours. For capnometric measurements, aprecision of ±0.43% CO₂+8% of the measured value is required accordingto the ISO 80601-2-55 standard. Devices on the market generallysignificantly exceed these requirements. Furthermore, a measuringinterval of 10 ms to 40 ms, preferably up to 25 ms, with measured valuesthat are independent from each other should be implemented for aresolution of the respiratory curve even with a higher respiratory rateof up to 150 breaths per minute which may occur inter alia withnewborns. Given this temporal resolution, it becomes possible toreliably determine the endexpiratory value, which is a medicalparameter. Especially for time-critical applications such as emergencymedicine, quick measuring readiness after a cold start is also desired,i.e., a short so-called startup time. In this regard, conventionalsystems sometimes require several minutes in order to reach thenecessary operating temperature. Moreover, economical and easyproduction is desired.

Filter wheel assemblies are known for example as marketed by MassimoInc. and for example disclosed in WO 03/060490 A. These assembliescomprise a mechanically rotating wheel in the optical beam path with atleast two filters for the gas absorption and the reference wavelength,and also with a region in which the light is completely blocked.Blocking allows a measurement of darkness that is used for offsetcompensation by the receiving unit. In this case, the measuringprinciple is realized that the measuring wavelength and the referencewavelength pass through the same optical path. The light also does nothave to be distributed to different paths. This is a time multiplexmeasuring method, and the use of mechanically moved components in theoptical beam path may be sensitive to vibration.

An assembly with spectral beam dividers is for example known from U.S.Pat. No. 5,464,982 A and U.S. Pat. No. 5,092,342 A, wherein modulatedlight is distributed at the receiving side to two or more detectors forthe gas absorption and for the reference wavelength. This is a typicalspectroscopy assembly as well, in which one light source is used and ahigh light yield is achieved since the light is split upwavelength-selectively rather than geometrically. Losses occur howeverfrom the damping in the transmission region of the filter. Simultaneousmeasurement occurs in the absorption wavelength and the referencewavelength so that there are no restrictions with regard to the timecharacteristic and the target quantity. It is comparatively involved tointegrate more than two wavelengths to measure several measuring gases.A beam divider filter may be necessary that can be expensive. For thesake of an offset compensation, the source can also be modulated in thiscase.

Another basic type of assembly is geometric parallel measurement. Inthis case, the light from the light source is guided to a plurality ofunfiltered receivers without beam division. For this, different opticallight paths are used for each measuring path, i.e., the gas path andreference path. This also means that in absorption measurement andreference measurement, different regions of the source and differenttransmission paths are used by the measuring devices. This results inambiguities as to which effect the geometric division and which effectthe actual absorption by the measuring gas has on the measurement. Theseambiguities can yield systematic measuring errors. These assemblies arefor example known from U.S. Pat. Nos. 4,618,771 A and 6,277,081 B1 andare used rarely in practice, for example by Goldwei (China). The verysimple assembly of simultaneously measuring absorption and reference andomitting mechanically moved components has advantages in this case.However, geometric parallel measurement suffers from worse suppressionof interferences with respect to spatial fluctuations by the source,different light transmission to the two detectors, for example by theformation of condensate in the cuvette, and by a reduced light yieldfrom the non-directional illumination of the receivers. In this case aswell, the source can be additionally modulated in the interest of offsetcompensation.

Another measuring principle includes a wavelength shift of the lightsource. The maximum of the emission spectrum of a usually narrowbandlight source is modulated alternatingly over time, usually thermallyinduced, between regions of a stronger and weaker absorption of thetarget quantity, such as CO₂. Accordingly, in the temporal sequence ofthe measured values, alternating values with a high influence on thetarget quantity (gas measurement) and with a slight influence (referencemeasurement) are available. Typically, lasers are used as a light sourcefor this type of measurement, such as diode lasers. In the field of CO₂climate measurement, a measuring procedure has become established inwhich an LED is spectrally modulated, as described in WO 2007/091043 A1,for example. This method only uses an optical path, a light source and adetector, and does not require an optical filter since the narrowbandlight source is spectrally modulated. These narrowband light sourceshave a low power consumption in comparison to the required emission;nonetheless, the thermal modulation of these light sources can also bevery energy-intensive. This is a time multiplex measurement, i.e., themodulation of the emission spectrum must be faster than the lowest timeconstant of the measured quantity. In capnometry, this restriction canbe problematic; in the field of climate measurement, this is not aproblem. Controlling, implementing the measuring method and producingthe sensors are very complex, in particular with LEDs and photodiodes.Furthermore, it is difficult to fill out the large temperature rangenecessary for modulation, and the measurement is vulnerable totemperature gradients. The light yield is reduced by the modulation,which negatively affects the signal-to-noise ratio (SNR).

In the case of capnometry, conversely, thermal emitters are primarilyused as a light source for optical CO₂ measurement in the middleinfrared (MIR) since the main absorption of CO₂ is at approximately 4.26μm. These thermal emitters are economical and robust. Light sources incapnometry assemblies should achieve a high emission performance toenable sufficient light intensity for a favorable SNR in the receivers.Detectors in the middle infrared range frequently have high thermalinherent noise; consequently, a strong useful signal is required for agood SNR. Likewise, a strong useful signal is needed in order to reachthe required concentration resolution for the given short measuring timeof 10 ms to 25 ms.

The physics of the thermal emitter dictate that the emitted intensityonly depends on the temperature of the emitter and other constants suchas the Stefan-Boltzmann constant and the emissivity. Increasing theoverall emitted optical performance can only be achieved by increasingthe emission area when using a maximum possible light source temperatureand with a highest possible emissivity. These parameters are technicallylimited. In practice, a large-area filament emitter or surface emitterwith an active emitting area of more than 1 mm² is typically used incapnometry at an emission temperature that is frequently 400° C. to 600°C. For example with an emission temperature of approximately 410° C.,the maximum emission is 4.26 μm, i.e., at the CO₂ absorption line. Inaddition, the corrosion of the thermal emitter is limited due to thiscomparatively low temperature, which is advantageous for the longevityof light source that is frequently not protected by an evacuated glassbulb. A glass bulb is omitted in order to prevent additional absorptionloss from the glass.

Such large-area emitters without an evacuated glass bulb however requirea great deal of electrical power in order to achieve the requiredemitter temperature, typically more than 300 mW with capnometers.Moreover, a large-surface emitter leads to a long startup time, forexample between 30 and 150 seconds in spite of an increase in powerafter the start.

The large thermal mass also makes the emitter sluggish so that only aslow modulation rate is possible with a time constant that is greaterthan that of the target quantity. Consequently, measurement isimpossible in AC mode for offset compensation, in particular when thebreathing is fast. The endexpiratory value is determined onlyimprecisely. With a thermally induced wavelength shift, it is difficultand expensive to implement the functionality of this measuring principlefor the ambient conditions in capnometry with regard to the temperaturerange and temperature gradient.

Another characteristic of thermal emitters (black bodies) is that theyhave a Lambertian radiation characteristic which, in combination with alarge emission surface, means that the light cannot be conductedefficiently and without loss to one or more detectors, even withbundling optical systems. A majority of generated radiation is thereforenot used for measurement, which is problematic in terms of energy.

Compensation of the thermal drift of the receivers and suppression ofthe influence of other thermal radiation sources on the measuringsignal, such as the housing temperature and sensor heating, can beachieved in the context of a so-called AC mode by amplitude modulationof the light while measuring transmission, or amplitude modulation isused that is generated by a filter wheel or chopper wheel. The lowmodulation frequency of large-surface thermal emitters is problematicwith electronic modulation since concomitant measurement is imprecise atfaster respiratory rates. Mechanical modulation is contrastinglymechanically liable to disruptions. In other embodiments, the light isnot modulated. This results in significant measuring errors from driftand parasitic thermal radiation. Such sensors are unsuitable forcapnometry.

In corresponding applications, thermopile detectors are generally used.These are large-surface in order to detect as much light as possible inthe structure. However, they are quite noisy so that the required signalfiltering leads to low limit frequencies of the detectors, whereby thefrequency in AC mode is also limited. This also affects the potentialrotation speed in a filter wheel structure. Alternatively, pyroelectricdetectors are routinely used that are however much more expensive.

SUMMARY

Contrastingly, the object of the present invention is to provide anassembly and a method for measuring a gas concentration, in particularfor the capnometric measurement of the proportion of CO₂ in breathingair, that improves use in the mobile sector and in the emergency sectorwith sufficient measuring precision and measuring speed.

This object is achieved by an assembly for measuring a gas concentrationby means of absorption spectroscopy, in particular for the capnometricmeasurement of the proportion of CO₂ in breathing air, in which IR lightis guided from a thermal light source through a measuring cell with agas mixture to be analyzed, and the gas concentration of a gas to bemeasured that is contained in the gas mixture is determined by measuringan attenuation of the light introduced into the measuring cell caused byabsorption by the gas to be measured, wherein the assembly has anoptical beam path with a thermal light source that generates IR light,the measuring cell that can be filled or is filled with the gas mixture,and a measuring path for the light generated by the thermal lightsource, one or more sensors as well as one or more bandpass filters thatare upstream from the one or more sensors, wherein the assemblyfurthermore comprises an evaluation apparatus that is designed todetermine the gas concentration to be measured from the attenuation ofthe IR light in the measuring cell, wherein at least one bandpass filteris designed to transmit within a measuring wavelength range in which thegas to be measured absorbs IR light, and at least one bandpass filter isdesigned to transmit in a reference wavelength range in which the gas tobe measured does not absorb IR light or only absorbs a slight amount incomparison to the measuring wavelength range, which is developed in thatthe thermal light source is designed as an encapsulatedmicro-incandescent lamp with a light-generating coil.

Within the context of the assembly according to the invention, these arequasi-punctiform light sources. Due to their low thermal mass, a highmeasuring rate is achievable, the startup time is significantly reducedin comparison to large-surface thermal emitters, and very little poweris required to achieve high spiral-wound filament temperatures. Giventhe low required power, the battery life for a corresponding measuringdevice, in particular a capnometer, is significantly increased. All ofthis ensures that a corresponding assembly is optimally suitable formobile use and in emergency medicine. Since the emission occurs within avery small geometric region, the loss of emitted light is also veryslight, so that a very high luminous efficiency is achieved on theemployed detectors which also further reduces the energy consumption.Accordingly, a small and lightweight light sensor with a long batterylife is feasible. Additional advantages are that no mechanically movedcomponents are used in the optical beam path, and a robust andeconomical measuring assembly is therefore achievable.

The basic concept of the present invention is that a small, punctiform,or respectively quasi-punctiform thermal light source is used with acomparatively high temperature and low-loss optical imaging of thesource on small, in particular quasi-punctiform detectors such asphotodiodes. The emission range is preferably much less than 1 mm². Withmicro-incandescent lamps, a spiral-wound filament, or respectively afilament within an area of for example 0.2 mm×0.5 mm can be wound, i.e.,an area of 0.1 mm² can be covered. Such a small coil is arranged, orrespectively encapsulated in a glass bulb that is evacuated or filledwith an inert atmosphere, which protects the coil from corrosion. Theencapsulation is necessary given the higher temperatures that may beused. This makes possible a high temperature of the thermal emitter, andconsequently a high light intensity within the MIR range as well. Thisfacilitates a good signal-to-noise ratio. Given the assembly in theevacuated glass bulb, a lower heat exchange between the emitter and theenvironment also results, and therefore lower power loss, as well asmechanical protection and a holder which is useful for production.

To minimize power loss from an excessive extension of the light source,it is advantageously provided that the encapsulation of themicro-incandescent lamp has a diameter of less than 2 mm, in particularless than 1.5 mm, in particular less than 1 mm. In addition oralternatively, the greatest linear distance between two points of thecoil is advantageously less than 1 mm, in particular less than 0.5 mm.By appropriately selecting the coil geometry, a very high emitted poweris achievable within a very small overall volume of the coil.Furthermore it is additionally or alternatively advantageous if anenvelope of the coil in a direction of projection in which the envelopeassumes a maximum envelope projection surface has a maximum envelopeprojection surface of less than 0.1 mm², in particular less than 0.02mm². This dimension that relates to the surface in a projection from aviewing direction in which the projection of the coil on the planeperpendicular to the viewing direction is at a maximum also ensures anadvantageous compactness of the light-generating coil.

Preferably, the sensor or sensors are designed as infrared-sensitivephotodiodes whose sensitive surface is in particular less than 1 mm², inparticular less than 0.15 mm². Such small-surface photodiodes have lessof a thermal noise load. Moreover, the light emitted by thequasi-punctiform light source can be focused very easily onto thesmall-surface infrared sensors by optical components. This yields a goodsignal-to-noise ratio. The components are furthermore economical androbust so that an economical and robust solution is also available withrespect to the sensors.

Typical useful photodiodes have a small detector surface, for exampleapproximately 0.35 mm×0.35 mm, i.e., a surface of 0.1225 mm² which is onthe scale of the radiating surface of the light source used according tothe invention. Optical point imaging can thus be realized therewith. Thesmall detectors have a high cut-off frequency so that the requiredmeasuring rate can be realized without any problems. In comparison tothermopiles and pyroelectric detectors, small photodiodes have a limitedspectral sensitivity and are thus less sensitive to interfering thermalradiation sources.

In one advantageous development, a control apparatus is included that isdesigned for power-controlled driving and/or modulation of themicro-incandescent lamp, wherein the control apparatus is designed toform a product from current measured at the micro-incandescent lamp andmeasured voltage in order to determine an actual value of the emittedpower, and/or is signal-linked to a photodiode arranged in themicro-incandescent lamp that receives a part of the light generated bythe micro-incandescent lamp. Even though many filament sources arenormally controlled by their voltage, voltage control, given thetemperature dependency of its inner resistance, is less suitable forproviding constant and reproducible power output than power control. Thepower consumption and the absorbed electrical power associated therewithas well as the emitted optical power are accordingly thus also dependenton the temperature of the spiral-wound filament. This is clearlyrevealed in particular when starting up the source after a cold startand when there are changes to the operating point. Accordingly, powercontrol of the filament source is recommendable for a so-called AC mode.

For power control, it is possible to electronically detect and multiplythe current and voltage of the radiation source and use this product asan actual quantity for control. A particularly fast control of the powerof the light source that is useful for modulating the light source canbe achieved by detecting current and voltage in an analog manner andmultiplying them in an analog manner. Accordingly, a purely analogcontroller can then also be used which has advantages in terms of speedand low cost. Alternatively, a digital realization of product formationand control is also possible.

In an alternate implementation of power control, part of the emittedradiation power is detected as free as possible from opticalinterference sources by means of a sensor such as a photodiode,preferably a low-noise economical VIS or NIR photodiode. Thephotocurrent generated in the photodiode is directly proportional to thedetected optical performance. Accordingly, the amplified signal of thephotocurrent can be used as an actual value for controlling the opticalperformance. This solution requires blocking out outside light as muchas possible. It must furthermore be ensured that a representativeportion of the emitted light is detected and used for controlling.

In a preferred assembly, the IR light is guided bundled through themeasuring cell and distributed to two or more sensors after passingthrough the measuring cell by means of a spectrally neutral opticalplane-parallel or curved transmission or reflection lattice, wherein thetransmission lattice or reflection lattice has in particular a latticeconstant that is less by a factor of 30 or more, in particular by afactor of 50 and more than a diameter of a light spot on thetransmission and reflection lattice. The “light spot” in this case isunderstood to be the light spot that is generated by the light which isemitted by the micro-incandescent lamp and passes through the opticalpath to the lattice. A lattice constant of 300 μm or less, in particularof 50 μm or less, is advantageous. This distribution by a spectrallyneutral lattice ensures that all the light of both the absorptionwavelength as well as the reference wavelength passes through the sameoptical path until being distributed and therefore does not suffer fromany systematic differences in absorption apart from the absorption to bemeasured in the measuring gas. By means of the spectrally neutrallattice, all of the light that has passed through the measuring cellindependent of the respective wavelength is then distributed into two ormore paths that are spatially separate from each other and is guided todetectors, or respectively detector units that are separate from eachother. Each detector unit can for example have one detector and oneoptical bandpass filter. These should have different spectralsensitivities so that the received a light on one detector is stronglydependent on the absorption of the measured quantity such as CO₂, andthe light received by the other detector, the reference detector, isessentially independent therefrom.

Furthermore, the spectral sensitivity ranges of the detector unitsshould be as close as possible to each other so that effects fromspectrally dependent, i.e., dispersive, broadband interferences such astemperature-dependent optical dispersion, or the spectral change of theemission characteristic of a thermal source caused by a temperaturechange, do not have an effect or only a scarcely different effect on thedetected signals. Interferences of the transmission in the optical paththat are broadband relative to the distance between the two sensitivityranges of the filters, broadband extraneous light and intensityfluctuations in the light source can be suppressed by offsetting thereceived detector signals against the absorption measurement signal.Accordingly, the precision of the measurement of the target quantity,such as the concentration of CO₂, is significantly increased.

It is advantageous in this context if the light components detected bythe detector units essentially originate from the same point of originof the light source, and essentially the same optical path is followedfrom the source until being distributed to the detector units. This isgenerally given to a large degree when using a small-surface filamentsource according to the invention. Moreover, the distribution of thelight should be essentially evenly distributed and without a strongspatial dependency, since otherwise the detected light componentsoriginate from different light paths. Accordingly, the source and pathinterferences act on the received light signals in the same way and canbe compensated when determining the measured quantity.

In the context of the present invention, a lattice that acts in aspectrally neutral manner is understood to be an evenly distributed,finely divided structure of an optical component in the beam path thatdeflects the direction of the incoming light in two or more directionstoward corresponding detector units. By means of this finely dividedstructure that is evenly distributed within the receiving aperture ofthe optical component, there is a diversified distribution of the lighttoward the detector units. This causes the light bundle to be dividedinto many small sub-bundles that are then distributed to the detectorsand detected there in a bundled manner. Interferences, whose spatialextent is greater than that of a sub-bundle, act on several sub-bundles,and therefore stochastically in the same way on the several detectorunits, and can be compensated in this way. In contrast, in extremecases, smaller interferences act selectively on only one detector path.The signal component of an individual sub-bundle which is affected bythis and the small interference that is contained therein is howeververy small in comparison to the overall signal so that the error in themeasuring signal that arises in this way can be neglected.

The structure of such a lattice should be as small as possible relativeto the entrance surface of the overall light bundle, i.e., the lightspot, so that the intensity component of a single sub-bundle relative tothe incident overall intensity is as small as possible. On the otherhand, the structure should be large enough so that wavelength-dependentdiffraction effects do not dominate the light distribution. Given awavelength in the middle infrared range of between 4 and 5 μm usefullyresults in a lattice constant of between 20 and 300 μm, preferably 200μm or less, more preferably 80 μm or less. The lower limit is about 20μm so that the diffraction effects are not too large. In addition, thedeflection angle should not lie within the range of the diffractionmaximum of the wavelengths used. For effective interference suppression,the surface components from the several directions should be essentiallythe same size, and the light intensities should therefore be evenlydistributed to the several resulting light paths. The cross-sectionalarea of the sub-bundles should have the same size so that interferencesacting in the beam path impact the sub-bundles in the same manner andcan therefore be compensated completely.

A corresponding spectrally neutral lattice can be designed as areflection lattice. In this case, the light is distributed andadditionally deflected, for example by 90°. This facilitates a verycomplex sensor design. The lattice can exist in a two-dimensional orthree-dimensional form that continuously expands in the third directionin space, for example in the shape of a sawtooth structure, in the shapeof a gabled roof structure, or more complex shapes such as a pyramidalstructure. Two or more directions of light distribution are therebypossible. In this manner, a good spatial distribution and separation ofthe light to two, three or more detectors can be achieved. By anadditional curvature of the entire lattice, such as in the form of aparaboloid, the focus of the incident light on the receiver cansimultaneously be moved without additional optical components beingnecessary for this, such as concave mirrors on the receivers.

Alternatively, the lattice can be a transmission lattice that is formedon or in a transparent material, in particular glass, which istransparent within the wavelength range of the measurement, inparticular within the IR range. In contrast to the reflection lattice, arefractive index difference of the substrate to the surroundings, orrespectively within the substrate, causes a change and distribution ofthe direction of the incident light. In this case as well, it isrecommendable to use two-dimensional or three-dimensional structuresthat provide additional focusing.

The combination of the lattice structure with focusing elements orstructures is characterized by easy positioning of the opticalcomponents in the beam path during sensor production. In particular,this relates to the transmission assembly.

For use within the aforementioned middle infrared range, a slant angleof a reflection lattice of, for example, approximately 27° is usefulthat achieves an angle between the detectors of about 100° with an exactdeflection of 90° without causing light components to be blocked by thelattice itself. By this spreading of the resulting light beams by about100°, it is possible to position the detector units in the proximity ofthe lattice, realizing a very compact structure. The deflection of thelight striking the lattice by precisely 90° is useful for adjusting andpositioning the detector units, and therefore for production.

Preferably, the measuring cell is designed as tubes that are diffuse orhave a high-gloss reflection inside, on one end of which themicro-incandescent lamp is arranged, and on the other end of which thesensor or sensors with the upstream bandpass filters are arranged. Thisfacilitates complete transmission of the emitted light and therebyincreases the measuring precision. A diffuse reflection, for example bymicrostructuring the reflective surface, causes an at least partialdecoupling of the light incoming to the receivers from the site oforigin and hence reduces systematic errors. A high-gloss reflection hasthe advantage of less light intensity loss. Gold is an exemplary,suitable material for internal mirroring that has good reflection in theIR range and is hardly subject to corrosion due to its nature as aprecious metal.

In an advantageous development of the assembly, the at least onebandpass filter is designed as a double bandpass filter that lets IRlight pass through both in the measuring wavelength range as well as inthe reference wavelength range, wherein the double bandpass filter isupstream from an individual sensor, wherein the control apparatus isdesigned and configured to modulate the micro-incandescent lamp betweenan operating point with a lower output and an operating point with ahigher output in which the respective emission spectrum has differentcomponent ratios in the measuring wavelength range and in the referencewavelength range.

This double bandpass assembly is based on the principle that the filterranges for gas absorption and the reference wavelengths are combined inan optical double bandpass filter. The light is blocked in all of theranges except for the two passthrough regions or bands for gasabsorption and the reference wavelength. This filter is located in theoptical path between the light source and the receiver. The preferablypower-regulated quasi-punctiform light source is alternatingly operatedat an operating point with lower power, or respectively lowertemperature, and an operating point with higher power. At the operatingpoint with lower power, the emission spectrum especially crosses therange of gas absorption. At the operating point with higher power, lightpasses through both filter ranges to the receiver. The detected light isaccordingly dominated by the absorption of the target gas at one pointin time, and an additional component is contained at the next point intime that includes the light transmission through the cell. Thistherefore includes the reference measurement. If the change in the gasconcentration within this time interval is negligibly small, theconcentration of the target quantity can be determined using themeasured values at the points in time of a high and low temperature, orrespectively power. In this case broadband transmission changes of theoptical path are suppressed like in a two-detector assembly, or byanother absorber. Only one receiver is necessary in this assembly.

The signal processing and evaluation for this instance will be brieflydescribed below. For a simplified description, it is assumed that thelight component of the reference range is negligible at the firstoperating point. The following then applies for the light intensityI_(AP1) arriving at the detector at operating point 1 (AP1):

I _(AP1) =k _(S) I ₀₁ e ^(−αxc),  (1)

where k_(S) is a spectrally independent damping factor that depictsinterference and constants which describe the amplification, the opticalimaging in the system and other influences, for example. The factor α inthe exponent designates the wavelength-dependent absorption coefficientof the measuring gas. The terms x and c describe the measuring path inthe measuring cell and the concentration of the measuring gas.

At the second operating point (AP2), the light intensity I_(AP2)arriving at the detector is:

I _(AP2) =k _(S) I ₀₁(k _(a) e ^(−αxc) +k _(b)).  (2)

In this case, the factors k_(a), k_(b) include the change in the lightintensity from the source in the filter range of the absorption signalat the transition from operating point 1 to operating point 2, orrespectively the relationship of the light intensity from the source inthe filter range of the absorption signal at operating point 1 to thelight intensity from the source in the filter range of the referencesignal at operating point 2. These factors thus also include the changein the intensity of the light source in the two windows at thetransition from operating point 1 to operating point 2. A conversionresults for the concentration, or respectively the target quantity c:

$\begin{matrix}{c = {k_{S}{I_{01}\left( \frac{\frac{I_{{AP}\; 2}}{I_{{AP}\; 1}} - k_{a}}{k_{b}} \right)}{\frac{1}{{- \alpha}\; x}.}}} & (3)\end{matrix}$

This measuring result strongly depends on the proportions of theintensities of the light source at operating point 1 and at operatingpoint 2. The practical evaluation is preferably carried out usingcalibrated curves or lookup tables since k_(a), k_(b) cannot bedetermined in a practical system during runtime.

The double bandpass filter assembly only requires one optical channeland one detector unit, resulting in a robust and optically simple designthat offers strong protection against optical interferences, changes inamplification and the sensitivity of the receiver. The optical powerprovided is not distributed to several receivers. The quasi-punctiformlight source can be precisely modulated by optical or purely electronicpower regulation which makes referencing, or respectively referencemeasurement, highly stable. The operating points can be checked by usingthe time characteristic of the detected signal during modulation.Additional advantages of this assembly are the low power consumption dueto the use of only one reception channel, and the low production costsdue to the low number of optical and electronic components.

In another advantageous development of the aforementioned assemblies,means are included that are configured for temporarily increasing thepressure and/or reducing the pressure of the gas mixture in themeasuring cell, in particular a pump and/or one or more in particularswitchable valves. Such valves can be switchable valves or other valvessuch as check valves or pressure relief valves. Preferably, a controlapparatus is included that is configured to control the means fortemporarily blocking a gas outlet of the measuring cell, and/or toincrease and/or lower the amount of the gas mixture to be analyzed inthe measuring cell. Alternatively, the pressure variation can also beused with the assistance of an external device in addition to the actualsensor module.

This measure advantageously represents a supplement to sensors based onconventional side stream measuring assemblies for sensor calibration,which is recommendable for longer intervals in time. A significantpressure change is used for referencing, for example from a largeincrease in pressure, or a variation of the pressure from anunderpressure to an overpressure in the measuring cell. To generate anoverpressure, a pump can be used for example in combination with a checkvalve in front of the measuring cell, wherein the flow direction ischanged, and the pump that otherwise draws gas out of the measuring cellcauses the gas to be compressed in the measuring cell. It may beeconomically sensible to use an external device that includes thenecessary hardware for performing the pressure manipulation. Such anadditional device is advantageously connected to the pneumatic inputsand outputs of the measuring cell. If the sensor system, or respectivelythe assembly, is designed for such an external calibration, the raw datacan be tapped, calibration data can be acquired, and updated calibrationparameters can be transmitted to the sensor. In this way it is possibleto economically design cyclically required calibration without expensivecalibration gases.

This measure primarily occurs in a side stream since the gas mixturedrawn into the side stream can be built up and manipulated in terms ofpressure comparatively easily. A nonlinearity of the absorption of thetarget quantity that goes beyond the law of absorption is thus used inorder to separate it from weaker and especially broadband backgroundabsorptions and thereby enable a selective and robust measurement of thetarget substance. An intensity drift of the source and changes in thetransmission of the transmission path are also suppressed in this methodwith respect to the target quantity. Since building up the measuring gasmeans an interruption in the actual measurement, it is a cyclicalmethod. It can be combined with continuous single channel measurement aswell as with multichannel measurement in order to effectively compensateinter alia for interfering effects.

The nonlinearity measurement is not limited to the application ofabsorption measurement with small-volume IR light sources. In thepresent case, it can be added to a previously described continuousmeasurement of a target quantity and determination of concentration. Thedetermination of the nonlinearity requires several measurements to bemade over the time period of the buildup in pressure, or respectivelythe change in pressure so that the nonlinearity can be deduced from therelationship between the measured values and development in pressure. Toaccomplish this, in the context of the nonlinearity measurement, theflow of the measuring gas is briefly blocked cyclically, preferably whenthere is a high concentration of the target quantity such as expiratoryCO₂. For this, drawing the gas to be measured can for example be stoppedfor a short time by for example reversing the flow direction of a pump,wherein the pump operates against a valve, in particular a check valve,in front of the measuring cell and thereby compresses the gas mixture inthe cell. If applicable, reducing the pressure is analogously possibleby throttling the valve before the measuring cell and retaining thedirection of flow of the pump. In this manner, the transmission of theassembly at different pressures of the gas in the measuring cell isdetected. If a significant change in pressure is reached in the cell,the pump restarts suctioning and continuous measurement restarts.

The change in pressure does not influence the light source, the lighttransmission and the receiver. These quantities can be assumed to beconstant over the duration of the change in pressure. Significantchanges, drift phenomena, aging effects, etc. have a greater timeconstant than the duration of the change in pressure and nonlinearitymeasurement. In contrast to optical and electronic transmission, theabsorption changes in the measuring cell with the pressure, however.Since the pressure of the gas in the measuring cell is also measured,the unknown transmission data of the measuring path can be determined inthis way.

By suitably designing the optical filter, the strength of thenonlinearity of the absorption of the target quantity can be influencedin conjunction with the optical gas absorption of the target quantity.Such a nonlinearity of the absorption does not occur in otheroverlapping, broadband absorbers that are generally weaker and also maybe included in the gas mixture. By using this property, the targetquantity can also be selectively determined without using an additionalreference wavelength measurement. The concentration value of the targetquantity ascertained in this way can be used to correct the valuesbetween the pressure cycles.

Physically, the nonlinearity results from the overlapping of thewavelength-dependent transmission characteristic of the filter with thewavelength-dependent absorption coefficient of the measuring gas thatunderlies the transmission of the light through the measuring cell andthe gas to be measured within the wavelength range of the optical filterand the absorption of the measuring gas. If the transmission window ofthe filter includes the tails of the absorption bands of the measuringgas, the absorption of the light passing through increases nonlinearlywith the increase in the concentration when the concentration of themeasuring gas is increased in the measuring cell. This is an effect thatexclusively results from the measuring gas to the extent that othergases in the gas mixture and other effects of the measuring apparatuscause an attenuation of the light that is largely independent of thewavelength within the corresponding spectral window of the filter. Onlyone transmission channel from a light source to a detector unit isrequired, wherein a “detector unit” is to be understood as a detectorand possibly an optical filter. The light from the source should beamplitude modulated since constant components must be suppressed. In thefollowing, the alternating signal amplitudes will be considered. Thedetected light power on the detector surface, or respectively the lightintensity I_(M) received by the detector can be described as:

I _(M) =I ₀ k _(S) T _(ges),  (4)

wherein I₀ is the light intensity emitted by the light source, k_(S) isa spectrally independent damping factor that includes interferences andconstants that describe the amplification, imaging, etc. in the system,for example, and T_(Total) describes the wavelength-dependenttransmission of the overall system. This factor is a product

T _(total) =T _(G) T _(S) T _(F)  (5)

of the wavelength-dependent and pressure-dependent transmission T_(G) ofthe target gas, the wavelength-independent and pressure-dependenttransmission T_(S) of interfering gases, as well as thepressure-independent and wavelength dependent transmission T_(F) of theoptical system from the optical filter, spectral sensitivity of thedetector, the spectral characteristic of the emission source as well asthe transmission properties of the cell. For the method, it is importantthat the transmission range of the bandpass filter is not just matchedwith the maximum of an absorption length of the target gas in a verynarrow band, but that ranges with low absorption are also included.Moreover, the filter passthrough band should be selected so thatinterfering gases have an essentially constant absorption characteristicin this region. Moreover, the filter bandwidth should be kept narrowtaking into account the stated considerations so that spectral, orrespectively dispersive, e.g., temperature-dependent, changes of thegenerally broadband light sources and detector have little to no effecton the measuring results.

Taking into account the pressure dependency and wavelength dependencyequation (4) can be described as a sum of the n spectral components:

$\begin{matrix}{{I_{M}(p)} = {\frac{I_{0}}{n}k_{S}{T_{S}(p)}{\sum_{n}{{T_{Gn}(p)}{T_{Fn}.}}}}} & (6)\end{matrix}$

After applying the law of absorption and taking into account thepressure dependency of the particle density in the measuring gas, thefollowing results:

$\begin{matrix}{{{I_{M}(p)} = {\frac{I_{0}}{n}k_{S}e^{{- \alpha}\; {xc}_{s}\frac{p}{p_{0}}}{\sum_{n}{e^{{- \alpha_{Gn}}{xc}_{G}\frac{p}{p_{0}}}k_{Fn}}}}},} & (7)\end{matrix}$

and expressing formula (7) as a logarithm:

$\begin{matrix}{{{\ln \mspace{11mu} {I_{M}(p)}} = {{\ln \mspace{11mu} \left( {\frac{I_{0}}{n}k_{S}} \right)} - {\alpha_{s}{xc}_{S}\frac{p}{p_{0}}} + {\ln \left( {\sum_{n}{e^{{- \alpha_{Gn}}{xc}_{G}\frac{p}{p_{0}}}k_{Fn}}} \right)}}},} & (8)\end{matrix}$

The logarithm of the measuring signal can thus be expressed as the sumof three terms of which one is pressure-independent, one is linearlydependent on pressure, and a third contains a component that isnonlinearly dependent on pressure. The linearly dependent componentcontains the interfering gas absorption, and the nonlinear componentcontains the target gas information. By means of further signalprocessing, these components can be separated from each other, therebyeliminating interfering influences in the measuring signal.

This can for example be accomplished by using the pressure derivative offunction (8), whereby the pressure-independent signal components areremoved from the measuring signal, and the nonlinear component resultingfrom the target gas remains as the only pressure-dependent component.Given the linearity of the pressure dependency of the potentialinterfering gas component, a constant value remains after thederivation:

$\begin{matrix}{{\frac{d}{dp}\ln \mspace{11mu} {I_{M}(p)}} = {{- \frac{\alpha_{s}{xc}_{S}}{p_{0}}} + {\frac{d}{dp}\ln {\left( {\sum_{n}{e^{{- \alpha_{Gn}}{xc}_{G}\frac{p}{p_{0}}}k_{Fn}}} \right).}}}} & (9)\end{matrix}$

The interfering gas component thereby forms an offset and can beseparated from the signal component of the target gas by considering thedifference in the signal at various operating points, or by a secondderivation according to pressure. The resulting signal progression isdependent on pressure in a nonlinear manner, although proportional tothe concentration of the target gas. The curve characteristic of thisnonlinear relationship, which can be advantageously saved as afunctional approximation or as a lookup table to calculate theconcentration in the signal evaluation, is essentially determined by theabsorption curve of the target gas and the chosen transmissioncharacteristic of the optical filter and can therefore be optimized forcertain concentration measuring ranges. A series of interferingquantities can be suppressed in this manner although only one measuringpath is used in the design. By recursive calculation, the signalcomponent of the interfering gas can then also be calculated, which forexample in capnometry provides an approximate indication of theconcentration of N₂O as the strongest interfering gas in capnometry.

Thus by using only one measuring path, a determination of concentrationis feasible which, apart from the SNR, is independent from thewavelength-stable transmission of light intensity from the source to thedetector within the optical filter bandwidth. This dispenses with a fewsources of error that for example arise from aging. A zero referencemeasurement, such as a cyclical measurement of ambient air isunnecessary in this method. Instead, the absolute concentration can bedetermined. Given the simple optical design, high light transmissionwith very low loss and consequently very good SNR is possible. Otherbroadband influences relative to the filter bandwidth such as otherabsorbers are effectively suppressed. Correspondingly, the assembly ismore robust against spectral fluctuations in the emission from thesource and the sensitivity of the detector in comparison to methods withtwo or more separate wavelength ranges for measuring gas absorption andthe spectral reference. This is because only the bandwidth of the filterin the absorption band is effective while referencing, and not thespectral distance between the absorption filter range and the referencerange. This method can be integrated to compensate for potential drifteffects in combination with existing side stream methods.

A different parameter from the aforementioned formulas can beintentionally changed in an analogous manner instead of modulating thepressure in the measuring cell, and the change of the measurement withrespect to its nonlinearity can be used. This applies for example to theabsorption coefficient α, the absorption length x or the concentrationc.

Likewise, the underlying object of the invention is achieved with amethod for measuring a gas concentration by means of absorptionspectroscopy, in particular for the capnometric measurement of theproportion of CO₂ in breathing air in an above-described assemblyaccording to the invention, in which IR light is guided from a thermallight source through a measuring cell with a gas mixture to be analyzed,and the gas concentration of a gas to be measured that is contained inthe gas mixture is determined by measuring an attenuation of the lightintroduced into the measuring cell caused by absorption by the gas to bemeasured, wherein the method is further developed in that the thermallight source is designed as an encapsulated micro-incandescent lamp witha light-generating coil, wherein in particular a sensor or severalsensors are designed as infrared-sensitive photodiodes with a sensitivesurface that is less than 1 mm², in particular less than 0.15 mm².

The method according to the invention accordingly achieves the sameadvantages, features and properties as the invention according to theabove described assembly. The method as well as the assembly canadvantageously be used both in the side stream method as well as in themain stream method.

Preferably, the thermal light source is modulated with a measurementrepetition frequency f_(Mess) that is greater than 10 Hz, in particulargreater than 25 Hz, wherein a temperature of the coil is greater than400° C. during measurement, and has a temperature modulation rise of atleast 300° C., in particular at least 500° C., in particular exceeds1000° C. at a maximum. The measurement repetition rate ensures that theprogression of the CO₂ concentration in the breathing air is sampledsufficiently even in the event of a fast respiratory rate. Highrespiratory rates occur in medical emergencies as well as in neonatalmedicine, i.e., in newborns and premature babies.

The use of high temperatures of the thermal light source ensures thatthere is sufficient intensity at a wavelength of about 4.26 μm, i.e.,within the absorption band of CO₂. At a maximum, the temperature ishowever significantly higher than is routine in capnometry, andsignificantly higher intensities are therefore available at themeasuring wavelengths. This is facilitated by selecting aquasi-punctiform filament source that can be encapsulated in a thinglass housing without a significant loss of light intensity.

The micro-incandescent lamp is preferably operated with power control.In this manner, very precise and reproducible controlling of the lightintensity and the emitted spectrum of the light corresponding to thetemperature of the spiral-wound filament can be realized, especiallywith the quasi-punctiform filament sources of the present inventionwhich exist in micro-incandescent lamps. Since the very smallspiral-wound filament has a very small thermal mass, hysteresis effectssuch as a temperature lag relative to the variably supplied power areacceptable, even with the required modulation frequencies. Furthermore,given the small thermal mass, a comparatively fast control and change intemperature of the spiral-wound filament is possible.

An efficient type of quasi-continuous suppression of interference anddetermination of the concentration results when, over the course ofmeasuring, the gas mixture pressure in the measuring cell is increasedand/or lowered sequentially over intervals in time and the absorption ismeasured depending on the pressure, wherein to change the pressure, inparticular, an outflow of the gas mixture is interrupted, and/or aninflow or an outflow of the gas mixture is supported and increased by apump. The dependency of the absorption in the gas mixture is non-lineardue to the tuning of the bandpass that lets through the absorption rangeof the gas to be measured to the absorption band of the gas to bemeasured. This non-linearity of the pressure dependency of theabsorption in the measuring cell is used to isolate the input of theamount of the gas to be measured from other inputs that do not yield acorresponding nonlinear input. The nonlinearity of the pressuredependency of the measured absorption therefore offers an independentreference of the absorption measurement.

A method having its own inventive value that can also be used in thecontext, but not just in the context, of the above-described assemblyaccording to the invention and the above-described method according tothe invention relates to a method for measuring a gas concentration bymeans of absorption spectroscopy, in particular for the capnometricmeasurement of the proportion of CO₂ in breathing air, in particularaccording to one of the above-described methods according to theinvention, in particular in an above-described assembly according to theinvention, in which light, in particular IR light, is conducted from alight source, in particular a thermal light source, through a measuringcell with a gas mixture to be analyzed, and the gas concentrations of agas to be measured that is contained in the gas mixture is determined bymeasuring an attenuation of the light introduced into the measuring cellcaused by absorption by the gas to be measured, characterized in thatover the course of measuring, the gas mixture pressure in the measuringcell is, at intervals in time, increased and/or lowered, or fluctuationsin the gas mixture pressure are measured, and the absorption is measureddepending on the pressure, wherein a pressure-dependent measuring seriesis analyzed with respect to components that are linearly and nonlinearlydependent on the pressure, and in particular the component that isnonlinearly dependent on the pressure is used to measure the gasconcentration of the gas to be measured, or to correct and/or calibratea measurement of the gas concentration of the gas to be measured.

The fundamentals of this nonlinearity measurement that can alternativelyalso be based on intentional changes in the absorption constant or theabsorption length have been described above in conjunction with theassembly using the above-cited formulas (4) to (9). This nonlinearitymeasurement can also be applied to gas absorption measurements withinvisible light or other wavelength ranges in addition to theabove-described context as long as the target gas has a narrowabsorption band on which an optical bandpass filter is placed incomparison to interfering influences.

Further features of the invention will become apparent from thedescription of embodiments according to the invention together with theclaims and the attached drawings. Embodiments according to the inventioncan fulfill individual features or a combination of several features.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below, without restricting the general ideaof the invention, based on exemplary embodiments in reference to thedrawings, whereby we expressly refer to the drawings with regard to thedisclosure of all details according to the invention that are notexplained in greater detail in the text. In the following:

FIG. 1 shows a schematic depiction of a first exemplary embodiment of anapparatus according to the invention for the side stream,

FIG. 2 a)-c) show schematic diagrams of beam paths for capnometry

FIG. 3 a), b) show schematic depictions of apparatuses according to theinvention with optically neutral transmission lattices,

FIG. 4 a)-f) show schematic depictions of optically neutral reflectionlattices that can be used according to the invention,

FIG. 5 shows a schematic depiction of a second exemplary embodiment ofan apparatus according to the invention,

FIG. 6 shows spectral characteristics of two operating points of a lightsource as well as a filter and absorption characteristic for acapnometry application,

FIG. 7 shows a schematic depiction of a third exemplary embodiment of anapparatus according to the invention,

FIG. 8 a), b) shows schematic depictions of power control circuits thatcan be used according to the invention,

FIG. 9 shows a schematic depiction of a fourth exemplary embodiment ofan apparatus according to the invention,

FIG. 10 shows a depiction of high-resolution spectral characteristics ofthe absorption band of CO₂ and a bandpass filter that can be usedaccording to the invention, and

FIG. 11 a)-d) show a schematic depiction of a micro-incandescent lamp.

In the drawings, the same or similar elements and/or parts are alwaysprovided with the same reference numbers; a reintroduction willtherefore always be omitted.

DETAILED DESCRIPTION

In FIG. 1, a first exemplary embodiment is schematically depicted of anassembly 10 according to the invention for the side stream. It can be acapnometer that comprises a measuring cell 12 which is supplied througha gas inlet 14 and a gas outlet 16 with a constant flow of breathing airby a pump (not depicted) that has been branched from a main flow ofbreathing gas of a patient. For this, the measuring cell 12 also has agas inlet opening 15 and a gas outlet opening 17 on its opposing side.The gas outlet opening 17 is preferably arranged entirely around themeasuring cell 12 so that the required cross-section of the gas outletcan be realized with a minimum distance between the measuring cell andfilter. The measuring cell 12 is designed cylindrical with a diameterthat is small relative to the length of the measuring cell 12.Accordingly, the volume of the measuring cell 12 is minimized relativeto the available absorption length, i.e., the length of the measuringcell 12. Minimizing the volume has the advantage that the measuring gasonly has a short retention time in the measuring cell 12, and thereforeenables very precise, high-resolution measurement over time of theconcentration of a target gas in the measuring gas.

For measurement, the assembly 10 comprises a quasi-punctiform filamentsource in the form of a micro-incandescent lamp 20 that is arranged inan evacuated glass bulb. This makes it possible for the light source toshine at a very high power and very high temperature without negativelyinfluencing its service life. The main portion of the emitted light liesin the infrared range for capnometry, in particular in the middleinfrared range. The micro-incandescent lamp 20 lies within the focalpoint of a spherical or parabolic reflector 22 that renders the lightbeams largely parallel so that the light shines through the measuringcell 12 as evenly as possible. The measuring cell 12 can be reflectiveon the inside.

An assembly consisting of two detectors 25, 27 with two upstreamfilters, 24, 26 and that are illuminated as evenly as possible by thelight that shines through the measuring cell 12 is located at the outputof the measuring cell 12. A filter 24 for a gas channel is locatedupstream of the detector 25 and has a narrow bandpass for the absorptionbands of the target gas, whereas the filter 26 is designed as a bandpassfilter for a reference channel where the target gas has no or onlyslight absorption. A control and evaluation unit is not depicted. Theuse of a quasi-punctiform light source in the form of amicro-incandescent lamp 20 makes it possible in this case to realizevery high measuring precision with very little light loss, and also toachieve very fast and precise power and temperature control of the lightsource.

Likewise, an evaluation apparatus 18 is symbolically depicted thatreceives signals from the detectors 25, 27 and ascertains theconcentration of the target gas in the measuring cell 12 according tointernal calculation rules, look-up tables, etc. and a correspondingcalibration.

For the sake of illustration, FIG. 2 a)-c) depict schematic diagrams ofbeam paths for capnometry. FIG. 2 a) shows the basic shape of a channelin which a quasi-punctiform infrared light source, in particular amicro-incandescent lamp 20, is arranged in the focal point of aparabolic reflector 22 that renders light beams emitted from the focalpoint of the reflector 22 a parallel light beam bundle. This parallellight beam bundle is in turn focused by a reflector 32, which is alsodesigned as a parabolic reflector, onto the focal point of the reflector32 in which a sensor 30 is arranged, which preferably also has a smallvolume, such as a photodiode. Only very little light is lost in thislight transmission so that a good signal-to-noise ratio (SNR) can beachieved with a relatively low initial intensity.

In contrast to this, the instance is depicted in FIG. 2 b) in which thelight source 20′ is not quasi-punctiform. In this case, part of thelight source 20′ lies outside of the focal point of the reflector 22 sothat the emitted light is not entirely transmitted as a parallel lightbundle to the opposing reflector 32, but rather passes the reflector 32in a nonparallel manner and is accordingly lost. Because of this lightloss, the amount and intensity of emitted light must be greater than inthe instance according to the invention, which leads to inefficiency andgreater power consumption.

FIG. 2 c) shows another example in which the light is also generated inthe same way as depicted in FIG. 2 a), however the parallel light bundlecontacts a schematically depicted, spectrally neutral transmissionlattice 40 that divides the light bundle without dispersive effects,i.e., spectrally neutral, into two different light bundles which aredistributed by the corresponding filters 24, 26 for a gas channel and areference channel 20 to two corresponding reflectors 32 and thecorresponding sensors 25, 27 for a gas channel and a reference channel.

FIG. 3 a), b) show schematic depictions of the optical conditions ofapparatuses according to the invention with spectrally neutraltransmission lattices. FIG. 3 a) shows an instance in which the light isgenerated as in FIGS. 2 a) and 2 c). After passing through the measuringcell 12, the parallel light bundle reaches a spectrally neutraltransmission lattice 41 that also focuses by means of a correspondingcurvature. In the shown embodiment, the parallel light bundle comes tobe by focusing the spectrally neutral transmission lattice 41 on threedifferent small-surface sensors 28, 28′, 28″ that for example can bedesigned for one reference channel and two different target gases withdifferent absorption ranges, alternatively also for three differenttarget gases, or also for one gas and two reference wavelengths whichthen enables dispersion compensation.

In the portrayed case, the light passes through the measuring cell 12perpendicular to the direction of flow of the gas mixture. This can beused both in a main stream as well as in a side stream. For use in aside stream, the coupling into, and respectively out of, the measuringcell 12 can however also be configured to be collinear with the mainstream direction in the measuring cell 12.

FIG. 4 a)-f) show schematic depictions of spectrally neutral reflectionlattices that can be used according to the invention. The structure 50from FIG. 4 a) shown in a cross-section serves to distribute the lightto two detectors; the structures 51, 52 from FIGS. 4 b) and 4 c) serveto split the light toward three detectors. FIG. 4 d) shows athree-dimensional, specific depiction of a reflection lattice 53 that,by its gable roof structure, serves to distribute to two detectorssimilar to the structure 50 from FIG. 4 a). FIG. 4 e) shows aperspective depiction, and FIG. 4 f) shows a front, plan view of asurface structure of a reflection lattice 54 that is based on a pyramidstructure with a hexagonal basic structure. This reflective lattice 54is suitable for distributing the light to six detectors.

FIG. 5 shows a schematic depiction of a second exemplary embodiment ofan assembly 110 according to the invention with a left partial image ina side view and with a right part in a frontal plan view following planeA:A from the left partial image. The micro-incandescent lamp 20 isenclosed in a holder that has a parabolic reflector 22 which convertsthe emitted light into a parallel beam bundle and shines through themeasuring cell 12 of the assembly 110 through which measuring gas flowsfrom the left (in the direction of the arrow). This assembly thereforecorresponds more to a measurement in a main flow; however, it can alsobe suitably used for a side stream measurement, or can be optimized fora side stream measurement by correspondingly coupling the light in andout in the measuring cell 12.

After exiting the measuring cell 12, the light bundle is deflected 90°in the shown view by a reflection lattice 53 corresponding to FIG. 4 b),wherein in this case a group consisting of a filter 24 for the gaschannel and a detector 25 for the gas channel of the target gas isdepicted as the detector unit, wherein the small-surface detector 25 isarranged in the focal point of the parabolic reflector 32. To protectthe individual components, three additional unidentified, opticallytransparent and spectrally neutral entry and exit windows are alsodepicted.

In the right depiction in FIG. 5, it is discernible how the reflectionlattice 53 divides the light bundle drawn as a circle into two differentlight bundles for two equivalently designed detector units for theabsorption channel and the reference channel. The reference numberscorrespond to those from FIG. 2 c) and FIG. 3 b).

An advantage of this design in the event of contaminants in the opticalpath is also shown in FIG. 5. Interference 112 is formed in themeasuring cell, for example a drop of condensation or a dirt particlethat is arranged fixed or movable in the measuring cell 12. Thereflection lattice 53 ensures that this interference is similarly inboth channels, i.e., in the absorption channel and the referencechannel, ensuring that such interference does not impair measurement.This can be seen in the right depiction in FIG. 5 where the images 114,114′ of the interference 112 appear in both channels in the same mannerand cause an attenuation of the signal in the same manner. Since theconcentration is measured by the absorption, i.e., the comparison ofintensities in the absorption channel and in the reference channel witheach other, both channels are affected in the same way. The structure ofthe images 114, 114′ is a result of distributing all the light to bothdetectors 25, 27 by the gable roof structure of the reflection a lattice53. Since both channels are affected, this input substantially cancelsitself out without substantially impairing the relationship between themeasured intensities.

FIG. 6 shows spectral characteristics of a light source at two operatingpoints as well as a filter and absorption characteristic for acapnometry application. The wavelength is depicted on the horizontalaxis within a range of 1 to 5 μm, i.e., the infrared range; on thevertical axis, normalized values are depicted for the emission,absorption and transmission between 0 and 1. According to the legend atthe bottom left, the CO₂ absorption of a 5% concentration and anabsorption length of 0.9 cm at about 4.26 μm are portrayed as a solidline. A range a) of a double bandpass filter is drawn around thisabsorption band that lets infrared light within a range of about 4.1 to4.4 μm pass through so that the entire range that is absorbed in CO₂falls within this range a) of the double bandpass filter. A second rangeb) of the double bandpass filter serves as a reference filter that letsinfrared light pass through approximately between 2.5 and 2.8 μm. Here,CO₂ does not possess any absorption.

The emission of a thermal emitter is depicted by a dot-dashed line and adashed line at a first and second operating point (AP) at 1300 K and 450K, respectively. At the higher temperature, the maximum of the lightintensity is at about 2.25 μm, whereas the portrayed characteristic hasnot yet reached its maximum with a thermal emitter temperature of 450 K.It should be noted that the emitted power is also greater at a highertemperature so that the two emission spectra in the depiction in FIG. 6have very different scaling factors from each other.

It is clearly discernible that at the lower temperature at operatingpoint 1, there is very little light intensity in reference band b),although there is much more intensity in absorption band a). At a higheroperating point 2 at 1300 K, the light intensity in the reference bandb) is however higher than in the absorption band a) so that, given aknowledge of the respective emission spectrum, a clear distinctionbetween the components of the reference and the gas absorption can bemade by means of a corresponding temperature modulation. This can bevery precisely adjusted within the framework of a calibration.

FIG. 7 shows a schematic depiction of a third exemplary embodiment of anassembly 210 according to the invention in a double bandpass assembly sothat the spectrum conditions from FIG. 6 apply in this case. The opticalassembly of one channel with which the measuring cell 12 is irradiatedcorresponds to the one from FIG. 2 a). This assembly can also be used ina main stream or in a side stream and, by suitably coupling in andcoupling out the light in the measuring cell 12, the light can be guidedin the direction of flow the measuring cell.

In the instance shown in FIG. 7, the light emitted by themicro-incandescent lamp 20 is filtered after passing through themeasuring cell 12 by a double bandpass filter 212 with the spectralcharacteristic from FIG. 6 and focused on the individual sensor 30. Forthis, a control apparatus 60 with a performance control is provided thatvery precisely controls the power output and temperature of thequasi-punctiform thermal emitter, i.e., the micro-incandescent lamp 20,in order to adjust the corresponding emission spectra from FIG. 6 tomodulate very quickly, or suitably select other temperatures with thecorresponding emission spectra.

FIG. 8 a), b) schematically depict power control circuits according tothe invention that can be used for this. A multiplication circuit isrealized in FIG. 8 a) in which a momentary current and a momentaryvoltage drop over the micro-incandescent lamp 20 are measured by twoamplifiers 68, 69, the current and voltage are multiplied with eachother in a multiplier 70 and entered into a control unit 62 as an actualvalue that is compared with a target value, and controls the lightsource via a series resistor as an output value (Out). The measurementof current and resistance and the multiplication can be either analog ordigital.

Alternatively according to FIG. 8 b), the light power emitted by thelight source 20 can also be measured by a photodiode 64 whose outputcurrent is in turn fed as an actual value via an amplifier 66 to thecontrol unit 62 that then correspondingly regulates the power of themicro-incandescent lamp 20 to the variable target value.

The solution according to FIG. 8 b) is simpler than that from FIG. 8 a),however it should be noted that a section of the spectrum of the lightsource is used for controlling the light source that does not serve tomeasure CO₂, but is instead proportional thereto. In addition, it isuseful to use a shortwave photodiode for control in comparison to themeasuring wavelength, for example within the visual spectrum or in thenear infrared since this enables much more stable control. Thesephotodiodes normally have a spectrum with a narrower band in comparisonto the emission spectrum of the micro-incandescent lamp 20 so that thetarget values must be correspondingly adapted. If applicable,nonlinearities should also be taken into account that arise from theoverlap of the respective temperature-dependent emission spectrum withthe sensitivity spectrum of the sensor.

FIG. 9 shows a schematic depiction of a fourth exemplary embodiment ofan assembly 310 according to the invention that can be linked to any ofthe above exemplary embodiments, i.e., to those exemplary embodimentsthat only have one optical channel as well as to those exemplaryembodiments in which the light bundle is divided into two separateoptical channels after passing through the measuring cell 12. Merely asan example, an assembly 10 according to the first exemplary embodimentfrom FIG. 1 has been selected as the basis for the assembly 310. Amodification exists in that the gas inlet 14 is equipped with a checkvalve 312 which opens in the gas inlet direction and blocks against thisdirection. At the output for the measuring cell 12 and the gas outletopening 17, a gas outlet 16 is attached that expands to a gas reservoir316 and is equipped with a pressure gauge 314 to measure the gaspressure in the gas reservoir 316, wherein the measured pressure alsocorresponds to the pressure in the measuring cell 12.

Furthermore, a pump 318 is provided that can pump gases either into thenarrow gas reservoir 316 and the measuring cell 12, or can be operatedin the reverse pump direction in order to conduct gas out of themeasuring cell 12 and thereby increase the pressure in one pumpdirection and lower it in the opposite pump direction. To do this, acontroller 320 for the pump 318 is provided that controls the pump rotorvia a motor 322 that can be designed as an actuator, or directly, andinfluences the direction and/or the strength of the pump 318. With thisarrangement 310, it is possible to perform a nonlinearity analysisdepending on the pressure available in the measuring cell 12 asdescribed above, for example according to formulas (8) and (9).Accordingly, it is inter alia possible to achieve a cyclically recurringpressure change in the measuring cell 12 that can be used as anindependent analysis in order to check whether the calibrationparameters of the underlying continuous measurement are still correct ormust be adapted since the concentration of the target gas can beisolated from interfering sources with the assistance of this method.Since this nonlinearity measurement has the best precision at hightarget gas concentrations, it is preferable to undertake pressuremodulation when for example, the end expiration value, typically about5% CO₂, has been reached during expiration.

The underlying spectral characteristics for this analysis are depictedin FIG. 10 in high resolution. The wavelength range between 4.18 and4.26 μm depicted on the horizontal axis corresponds to a narrow sectionfrom the spectrum range depicted in FIG. 6. The band structure of CO₂absorption can be clearly seen, in this case a depiction oftransmission. In contrast to the double bandpass filter from FIG. 6, anarrower bandpass filter is used in this case that cuts off the edgeregions of the absorption of CO₂ and is thus comparatively slightlynarrower than the CO₂ absorption band. A bandwidth that is too smallwould undesirably reduce the available light intensity; however, verystrong nonlinearity in the pressure dependency is ensured with the shownconfiguration. The intensity arriving at the detector results, from amathematical perspective, from an overlapping of the filter functionwith the wavelength-dependent function of the target gas absorptioncoefficient with its band structure. This overlap causes a nonlineardependency of the detectable transmission on the pressure, orrespectively the concentration of the target gas.

Also drawn is an interfering gas absorption coefficient that isbasically assumed to be constant in the depicted range and which alsoreduces the transmitted light power. Since however the absorptioncoefficient of the interfering gas is wavelength-independent, theoverlapping of the filter function of the interfering gas transmissionin this range only yields linear and no non-linear pressure-dependentterms.

FIG. 11 a) schematically depicts a micro-incandescent lamp 20 as anexample that can be used according to the invention. This comprises acoil 81 as a central light generating unit which is arranged in acapsule 84 that is evacuated or filled with an inert gas. The areaaround the coil 81 is provided with a vacuum 85. The capsule 84 itselfis transparent, for example made of glass. The evacuated interior issealed at the bottom by a solid base 87 which is penetrated by twosupply conductors 86 that terminate in contact pins 88 which the coil 81contacts. The capsule 84 has a basically cylindrical shape with adiameter 90 in relation to the longitudinal axis of themicro-incandescent lamp 20. This is advantageously less than 2 mm,wherein even smaller capsules 84 are even more advantageous foroptimally exploiting power.

The coil 81 has a curved shape so that a large amount of coil length isavailable in a relatively small space, and a high power density isaccordingly achieved. FIG. 11 b) shows the coil 81 together with itsenvelope 83, wherein a distance is left in the schematic depictionbetween the envelope 83 and the coil 81 only for the sake ofillustration. The envelope surrounds the volume 82 that the coil 81assumes. The greatest linear extension of the envelope 92 runs betweenthe two endpoints of the coil 81 in this case and hence is mostlyoutside of the envelope 83 of the coil 81. The greatest linear extension92 however also describes the greatest, or respectively greatestpossible linear distance between two points of the coil 81.Consequently, this greatest linear extension 92 is a linear measure ofthe compactness of the light-generating region of the micro-incandescentlamp 20. Especially in an imaging optical system, a very small greatestlinear extension 92 is advantageous because a majority of the generatedlight can be arranged in the focal point of the imaging optical systemso that light loss can be avoided. This translates to the benefit of thesignal-to-noise ratio and the battery life.

At the same time, a corresponding coil 81 has a very small thermal massso that the coil is heated to the operating temperature within fractionsof a second, and a modulation of several hundred ° C. is feasible givena sufficient frequency for a time-resolved measurement of the change ofa measuring gas concentration, for example for tracking theconcentration of CO₂ in a breathing gas in the context of capnometry.

FIG. 11 c) shows the coil 81 from the narrow side. In this case, thethickness of the coil is the same as the smallest linear extension 94 ofthe envelope 83. FIG. 11 d) shows an alternative coil 81′ that, incontrast to the coil 81, is straight and not bent. The envelope 83′ inthis case is basically cylindrical, and the greatest linear extension ofthe envelope lies within the coil 81′. While the absolute length of thecoils is the same, the linear extension of the coil 81′ is thereforegreater than in the case of the coil 81 presented beforehand. A bend inthe coil is therefore advantageous in the context of the presentinvention in order to generate high light intensity from a small overallvolume.

The above-presented exemplary embodiments each present the ideal case ofan assembly for example with division by a transmission lattice or areflection lattice that is always spectrally neutral, a double bandpassfilter assembly, and an assembly for the nonlinear analysis. Theseassemblies can however also be combined with each other so that thenonlinear analysis can for example also be used in a double bandpassfilter assembly, or an assembly with a spectrally neutral lattice and aplurality of receivers. Likewise, a double bandpass filter assembly canbe combined with spectrally neutral transmission lattices or reflectionlattices and a plurality of detectors in order for example to perform ananalysis with respect to a plurality of target gases.

All named features, including those taken from the drawings alone aswell as individual features that are disclosed in combination with otherfeatures, are considered, alone and in combination, to be essential forthe invention. Embodiments according to the invention can be fulfilledby individual features or a combination of several features. In thescope of the invention, features which are designated by “in particular”or “preferably” are understood to be optional features.

LIST OF REFERENCE SIGNS

-   -   10 Assembly    -   12 Measuring cell    -   14 Gas inlet    -   15 Gas inlet opening    -   16 Gas outlet    -   17 Gas outlet opening    -   18 Evaluation apparatus    -   20 Micro-incandescent lamp    -   20′ Expanded light source    -   21 Evacuated glass bulb    -   22 Reflector    -   24 Filter for gas channel    -   25 Detector for gas channel    -   26 Filter for reference channel    -   27 Detector for reference channel    -   28, 28′, 28″ Detectors    -   30 IR photodiode    -   32 Reflector    -   34 Lost light components    -   40 Spectrally neutral transmission lattice    -   41 Spectrally neutral focusing transmission lattice    -   42 Spectrally neutral transmission lattice    -   50-54 Spectrally neutral reflection lattice    -   60 Control apparatus    -   62 Control unit    -   64 Photodiode    -   66 Amplifier    -   68 Amplifier for current measurement    -   69 Amplifier for voltage measurement    -   70 Multiplier    -   81 Coil    -   82 Volume    -   83, 83′ Envelope    -   84 Capsule    -   85 Vacuum    -   86 Supply conductor    -   87 Base    -   88 Contact pin    -   90 Diameter of the encapsulation    -   92 Greatest linear extension of the envelope    -   94 Smallest linear extension of the envelope    -   110 Assembly    -   112 Interference    -   114, 114′ Image of the interference    -   210 Assembly    -   212 Double bandpass filter    -   310 Assembly    -   312 Check valve    -   314 Pressure gauge    -   316 Gas reservoir    -   318 Pump    -   320 Controller for the pump    -   322 Motor

1. An assembly for measuring a gas concentration by means of absorptionspectroscopy in which IR light is guided from a thermal light sourcethrough a measuring cell with a gas mixture to be analyzed, and the gasconcentration of a gas to be measured that is contained in the gasmixture is determined by measuring an attenuation of the lightintroduced into the measuring cell caused by absorption by the gas to bemeasured, wherein the assembly has an optical beam path with a thermallight source that generates IR light, the measuring cell that can befilled or is filled with the gas mixture, and a measuring path for thelight generated by the thermal light source, one or more sensors as wellas one or more bandpass filters that are upstream from the one or moresensors, wherein the assembly furthermore comprises an evaluationapparatus that is designed to determine the gas concentration to bemeasured from the attenuation of the IR light in the measuring cell,wherein at least one bandpass filter is designed to transmit within ameasuring wavelength range in which the gas to be measured absorbs IRlight, and at least one bandpass filter is designed to transmit in areference wavelength range in which the gas to be measured does notabsorb IR light or only absorbs a slight amount in comparison to themeasuring wavelength range, wherein the thermal light source is designedas an encapsulated micro-incandescent lamp with a light-generating coil.2. The assembly according to claim 1, wherein the encapsulation of themicro-incandescent lamp has a diameter of less than 2 mm, less than 1.5mm, or less than 1 mm.
 3. The assembly according to claim 1, wherein agreatest linear distance between two points of the coil is less than 1mm, or less than 0.5 mm.
 4. The assembly according to claim 1, whereinan envelope of the coil in a direction of projection in which theenvelope assumes a maximum envelope projection surface has a maximumenvelope projection surface of less than 0.1 mm², or less than 0.02 mm².5. The assembly according to claim 1, wherein the sensor or sensors isor are designed as infrared-sensitive photodiodes whose sensitivesurface is in particular less than 1 mm², or less than 0.15 mm².
 6. Theassembly according to claim 1, wherein a control apparatus is includedthat is designed for power-controlled driving and/or modulation of themicro-incandescent lamp, wherein the control apparatus is designed toform a product from current measured at the micro-incandescent lamp andmeasured voltage in order to determine an actual value of the emittedpower, and/or is signal-linked to a photodiode arranged in themicro-incandescent lamp that receives a part of the light generated bythe micro-incandescent lamp.
 7. The assembly according to claim 1,wherein the IR light is guided bundled through the measuring cell anddistributed to two or more sensors after passing through the measuringcell by means of a spectrally neutral optical plane-parallel or curvedtransmission or reflection lattice, wherein the transmission lattice orreflection lattice has in particular a lattice constant that is less bya factor of 30 or more, or by a factor of 50 and more than a diameter ofa light spot on the transmission and reflection lattice.
 8. The assemblyaccording to claim 1, wherein the measuring cell is designed as tubesthat are diffuse or have a high-gloss reflection inside, on one end ofwhich the micro-incandescent lamp is arranged, and on the other end ofwhich the sensor or sensors with the upstream bandpass filters arearranged.
 9. The assembly according to claim 1, wherein the at least onebandpass filter is designed as a double bandpass filter that lets IRlight pass through both in the measuring wavelength range as well as inthe reference wavelength range, wherein the double bandpass filter isupstream from an individual sensor, wherein the control apparatus isdesigned and configured to modulate the micro-incandescent lamp betweenan operating point with a lower output and an operating point with ahigher output in which the respective emission spectrum has differentcomponent ratios in the measuring wavelength range and in the referencewavelength range.
 10. The assembly according to claim 1, furthercomprising means that are configured for temporarily increasing thepressure and/or reducing the pressure of the gas mixture in themeasuring cell, in a pump, and/or one or more switchable valves.
 11. Amethod for measuring a gas concentration by means of absorptionspectroscopy according to claim 1 in which IR light is guided from athermal light source through a measuring cell with a gas mixture to beanalyzed, and the gas concentration of a gas to be measured that iscontained in the gas mixture is determined by measuring an attenuationof the light introduced into the measuring cell caused by absorption bythe gas to be measured, characterized in that the thermal light sourceis designed as an encapsulated micro-incandescent lamp with alight-generating coil, wherein in particular a sensor or several sensorsare designed as infrared-sensitive photodiodes with a sensitive surfacethat is less than 1 mm², or less than 0.15 mm².
 12. The method accordingto claim 11, wherein the micro-incandescent lamp is modulated with ameasurement repetition frequency f_(Mess) that is greater than 10 Hz orgreater than 25 Hz, wherein a temperature of the coil is greater than400° C. during measurement, and has a temperature modulation rise of atleast 300° C., or at least 500° C., or exceeds 1000° C. at a maximum.13. The method according to claim 11, wherein the micro-incandescentlamp is operated with power control.
 14. The method according to claim11, wherein over the course of measuring, the gas mixture pressure inthe measuring cell is increased and/or lowered sequentially overintervals in time and the absorption is measured depending on thepressure, wherein to change the pressure, in particular, an outflow ofthe gas mixture is interrupted, and/or an inflow or an outflow of thegas mixture is supported and increased by a pump.
 15. The method formeasuring a gas concentration by means of absorption spectroscopyaccording to claim 11, in an assembly in which light is conducted from alight source through a measuring cell with a gas mixture to be analyzed,and the gas concentrations of a gas to be measured that is contained inthe gas mixture is determined by measuring an attenuation of the lightintroduced into the measuring cell caused by absorption by the gas to bemeasured, characterized in that over the course of measuring, the gasmixture pressure in the measuring cell is increased and/or loweredsequentially over intervals in time, or fluctuations in the gas mixturepressure are measured, and the absorption is measured depending on thepressure, wherein a pressure-dependent measuring series is analyzed withrespect to components that are linearly and nonlinearly dependent on thepressure, and in particular the component that is nonlinearly dependenton the pressure is used to measure the gas concentration of the gas tobe measured, or to correct and/or calibrate a measurement of the gasconcentration of the gas to be measured.
 16. The assembly according toclaim 1 wherein the measuring gas concentration is capnometricmeasurement of the proportion of CO₂ in breathing air.
 17. The method ofclaim 11 wherein the measuring gas concentration is capnometricmeasurement of the proportion of CO₂ in breathing air.
 18. The method ofclaim 15 wherein the measuring gas concentration is capnometricmeasurement of the proportion of CO₂ in breathing air.
 19. The method ofclaim 15, wherein the light is IR light.
 20. The method of claim 15,wherein the light source is a thermal light source.